This invention relates to power amplifier circuits, and more specifically to a linear amplifier which minimizes power loss in its output devices. Originally optimized for driving an inductive load, namely a magnetic deflection yoke, the present invention is a power-efficient operational amplifier suitable for driving either resistive or reactive loads.
It is well known that the power supply for an amplifier which directly drives a load must have a supply voltage at least slightly greater than the maximum output voltage required by the load. The difference between the supply voltage and the instantaneous load voltage appears across the amplifier's output devices. Because load current also flows through the output devices, there is a consequent power loss through them. If to a first order, the supply current is nearly equal to the load current, then the efficiency of the amplifier, while driving a resistive load, is given by the following formula: ##EQU1## From the above relationship, it is evident that efficiency increases as the load, or output, voltage approaches the supply voltage. In most amplifier applications, however, the load voltage varies and is less than maximum the majority of the time. Therefore, efficiency suffers the majority of the time as well. The common technique of using a class A amplifier powered by a supply voltage high enough to accommodate the highest expected output voltage exhibits this characteristic.
It has been recognized in the prior art that for an amplifier to drive a load efficiently at both high and low output voltages, the amplifier must be able to supply load current from a low voltage supply for low output voltages and from a higher voltage supply only during higher output voltages. This reduces the voltage drop across the amplifier output devices and, thus, improves its efficiency. Prior art techniques of multiple power supply operation include circuits similar to that illustrated in FIG. 1. These circuits use multiple output devices connected so that their inputs are effectively in parallel yet load current flows through them in series. For example, load current flows through only one output device from power supply V.sub.1 for V.sub.out &lt;V.sub.1, through two output devices from power supply V.sub.2 for V.sub.out &gt;V.sub.1, and so on. When these circuits operate from a power supply higher than the lowest, the input current of such amplifiers characteristically must increase by the quantum amount necessary to support conduction of an additional output device. A further quantum increase must occur when operating from a third supply higher in voltage than the second, and so on. Each increase in input current makes greater demands on the circuit driving the output devices and may cause distortion in the output signal during the shift from one power supply to another.